Spurs looked the most dangerous of the two sides in the opening exchanges as they twice went close to taking the lead.
Grant Ward nearly opened the scoring when he struck the post after an excellent move from the young Spurs side.
Tottenham goalkeeper Lawrence Vigouroux kept the scores level when he pulled off a save just before half time.
Dendoncker gave Anderlecht the lead after 58 minutes when he finished off a counter-attack by the Belgian side.
Heylen doubled Anderlecht's advantage with seven minutes remaining when he was quickest to react when an Anderlecht penalty was saved by Vigouroux.
The result means that both sides qualify for the last 16 of the competition.